doc: make example more clarified in cluster.md

PR-URL: #23931
Refs: #23930
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/cluster.md

@@ -198,8 +198,8 @@ Within a worker, `process.on('message')` may also be used.
 
 See [`process` event: `'message'`][].
 
-As an example, here is a cluster that keeps count of the number of requests
-in the master process using the message system:
+Here is an example using the message system. It keeps a count in the master
+process of the number of HTTP requests received by the workers:
 
 ```js
 const cluster = require('cluster');